In the medical field, there have been used transportable radiographic image capturing apparatuses such as FPDs (Flat Panel Detectors), which detect intensity of a radiation transmitted through a human body to capture an image of an inside portion of the body. The FPD (hereinafter referred to as the electronic cassette) can capture the image while keeping a patient on a bed or the like, can be moved to adjust an area to be captured, and thereby can be flexibly used also for immobile patients.
In the electronic cassette, even if the radiation is not emitted, electric charges are generated by a dark current and stored in each pixel. The dark current appears as noise in the radiographic image. Therefore, in general, in the electronic cassette, a procedure of removing electric charges stored in each pixel is repeatedly carried out before a process of capturing a radiographic image. In a case where the radiographic image capturing process is performed, a control unit sends an image capturing instruction to the electronic cassette and a radiation apparatus for emitting the radiation. In a case where the radiation apparatus receives the image capturing instruction, the radiation apparatus starts to emit the radiation, and the electronic cassette starts to be exposed. When the radiation emission is completed, the electronic cassette reads the electric charges accumulated by the emission. In this process, the radiation emission timing of the radiation apparatus and the exposure timing of the electronic cassette are synchronized. Thus, the image capturing timings are synchronized.
Japanese Laid-Open Patent Publication No. 2010-081960 describes a first time measurement means disposed in a console (the control unit) and a second time measurement means disposed in the electronic cassette. The first and second time measurement means are synchronized with each other. At an exposure start predetermined by the console, the radiation is emitted from the radiation apparatus for a predetermined time. After the predetermined time has elapsed from the exposure start, the electronic cassette reads the electric charges generated in a radiation detector.
Various types and specifications of the electronic cassettes are used depending on the requirements of the image capturing conditions and the like. Also, the electronic cassettes are relatively costly. Therefore, as is often the case, a plurality of the electronic cassettes are not placed in each image capturing room, but are shared by a plurality of the image capturing rooms. In a case where a plurality of the electronic cassettes are shared by a plurality of the image capturing rooms, a user (such as a radiation technician) may make a mistake in selecting the electronic cassette. In the image capturing process, the electronic cassette has to be switched from a sleep mode to an image capturing mode in response to the instruction sent from the control unit (such as a console or a system controller). Therefore, in a case where the user makes the selection mistake, the image capturing process is performed while the electronic cassette remains in the sleep mode, thereby failing to obtain the radiographic image. Thus, Japanese Laid-Open Patent Publication No. 2009-219586 describes a technique, in which the image capturing process can be performed even if the user makes the selection mistake.
In Japanese Laid-Open Patent Publication No. 2009-219586, considering the possibility of the selection mistake, all the electronic cassettes (1) are switched from a standby mode to a capturing mode in the radiographic image capturing process. Thus, all the cassettes and their radiation detection means (22), located outside the subject area, are made ready for the radiation detection. Then, when the radiation detection means of one electronic cassette detects the radiation, the other electronic cassettes are returned to the standby mode (see, abstract, FIG. 8, and paragraphs 0054 to 0065 of the document).